feat: drag & drop for Pipeline Kanban columns (@dnd-kit)

Cards can be dragged between all 7 stages. Drop target highlights with
a dashed border + tinted background. A floating card overlay follows
the cursor during drag. Click-to-select still works (fires only when
no drag occurred, guarded by activeId check).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -10,6 +10,9 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@dnd-kit/core": "^6.3.1",
+    "@dnd-kit/sortable": "^10.0.0",
+    "@dnd-kit/utilities": "^3.2.2",
     "@emotion/react": "^11.14.0",
     "@emotion/styled": "^11.14.1",
     "@mui/icons-material": "^9.0.1",